Hydraulic pressure intensifier unit



Dec. 14, 1948'. y G; R. ELLIOTT 2,455,984

HYDRAULIC PRESSURE INTENSIFIER UNT Filed Jan. 22, 1945 5 Sheets-Sheet ly "WQ/maw Dec. 14, 1948.A 5.` R. ELLIOTT 2,455,984 HYDRAULIC PRESSUREINTENSIFIER UNIT l Filed Jan. 22, 1'945 3 Sheets-Sheet 5 INV-ENTO Gama/yf?. 2f/077 ,QTTORNEY v. 52 vncuun Patented Dec. 14, 1948 4 'STATESPATENT oFF-icc y 2,455,9sigjh`iij..

HYDRAULIC PRESSURE iNTENsmrER-:UNIT 1 G6 dan R. Ellioterernaaxe, Mich.,assigner. to

wankee, Wis.

Irving A. Puchner and Edward U.' DenimergvMiL" vAppl,nation January 122,1945,;serial Nogstsgce? f 9 claims. (c1. sof-'545) This inventionrelatesto hydraulic unit for the actuatiunof. Vehicle# .or'truclgfbrakes Moreparticularly the invention relates to thec'ombinatio'n with thehydraulic master .cylinder manually operable..brakes, .ot-.apowerunit-'for electing a seconda-ryl powerbrak operation, as shownsimilarly.in .co-.pending ,application Serial No'. v508,033, iiled .October 28,1943,. new, Patent No. 2,440,654.

" ItJis. the. object. oilthis .invention to. provide in conjunction.with.. the...'conyentonal vhydraulic l master liluid cylinder. governingvehicle braking,y a secondarypW breaking .mechanism including..'a1,.diaphragm .actuating ,.powerf chamben a hydrauliccylinder.. and avacuum-airfcontrolling Valve...

v Itis.. the. objectv of.this,.invention.to provide a secondary powerlbrakelmechanism characterized by. a manifold `vacuum[suspended? powerchamber actuated .y diaphragm, togethrl. with l means .for disconnectingvacuumto. one of ther; sides. thereof and ..a`dmitting air-.. thereto.for.4 actuation and translation.'.tl'iereofn f. Y A

' It.. is .the .further 7 .object herein to.; provision a brake.-operating hydraulic. cylinder .forsupplying to'. .the .vehiclelbrakesin-tially .manually operable master. cylinderbrake-.l-uid. under.pressure, andA secondary power. actuated fluid .under pressure.

The invention. relates. to -.tlnevarious arrangement...of elements and.theirl combinations a-s` more fullyset out in connection with.theappended drawings,.ofwl1ich.-.- Y n l Fig'. ,1 isja diagrammaticviewshowing-.the hydraulic vehiclev .brake ,'actuating-. cylinder, the.manual-.masten .cylinder .brake mechanism,. and the.' .supplementary.`power brake-v-.unit vjoinedthereto.;` n

Fig; 2 isan, .enlarged .elevationalY section of the hydraulic brake).cylinden andvpower brake-unit.

Fig'. 3 is an elevation. viewvfthereoiwith remote power. chamber.. n

y The above. .drawings .are `:merely illustrations of a prefer-ableembodimenti'of-:the presentinvention it 'being .understood that otherembodiments are,` contemplated within the scopeofthi-s invention assetbutfinA .the appended claims.

` InFifg; 1 "suitablohydrauliclwheel cylinders l are provided adjacent.the vehicle wheel brakes; 2 withhydraulic connections theretobeingsup-vplied throughfconduits yfrom hydraulic cyl- The hydraulic; powerrunit 9with powerv chamber I!) 'is carriedbybolts l I` secured to the vehicleframe I2y or suitable bracket secured*k thereto. The vacuum hydraulicvalve housing.l3 vwith air inlet-s" lll'isjoined tovehicle intake vacuummanio'ld` vI5"by conduit 'i6 and-manifold check valve l'l' for supplying.vacuum fto power chamber `Ill as hreunderexplained.

Referring .to F`g.f2 lthe vehicle brakehydraulic cylinder 4v has anoutlet port I8 for communication lwith hydraulic brake lines 3 shown inFig'. 1. VVK-rile 011.@ Suchfport 'is shown in Fig. 2, Fig.. 1 shows twosuch outlet ports i8 'for communication with'conduits y' y intake portliaconnects with conduit s from the'` hydraulic master cylinder 1whereby manually operated brake fluid is supplied under pres,- sure tohydraulic'cylinder 4 tobedirected through the longitl'ldirar'opening2Biwithin pi-ston 2| slidingly positioned -incy1inder 4. PistoneXpansion-cup ZZfcarried. by .piston 2l kprovide an effectiv'e hydraulicvs'al within cylinder 4.

Power chamber IU slprovided at one side thereof fwith'a vacuum inletconnection 23 adapted forloonnecti'on yto vacuum conduit i6' yfrom theVehicle' intake manifold l5. inthe manner illustratedin nigga.' l

The .Vacuun'if valve body housing 24 extending tivi-thin`V power.chamber housing lll is suitably secured Ito' the annular ange 25 ofcylinder` housing 4 by bolts 26'and nuts 21. It will be noted thatl saidYflanged portion'25 also forms a part of `airand vacuum Valve body 24.Power chamber In" 'is likewise retained inxed relation to valve ,fbody`24 by vbolts-2h' asghcwnv in Fig. 2.

' A hollow aircontrol diaphgram .28 is-periplfier-v allyr'etainedbetween the annularly anged members 24I and 25,-which forinthe controlvalve body... Hollow diaphragm stiifening plate 29 is secured on ,oneside of saidv diaphragm by rivetsv 30,' the' other endsof saidrivetsretaining air valve control disc 3|. onthe other side of said diaphragm..Y n

. 'Disp ,3l lis punched outwardlyto form the annular flangednembe'r-BZ-which is adapted to unseat aircontrolvdisc533, the latterbeing norin ally.retained'byI spring-34 over annular airOpeningSSin--airfvalve seat-36; It will benoted secured. between valvelbody ange 24 and diaphagm .28'. Y

HollOW li'langedgvalve control piston 31 is reciprocably-.and -slidably`positioned within the end of hydraulic-cylinder 4 in engagement-withspring4 38., the-otherendof which is -seated against As shown in Fig. 2,vacuum is established inY` chamber 44 through openings 45 in hollowpower rod 46. It will be noted that. movement to the left of valvecontrol piston 31 causes disc. 41v thereon to sealingly engage plate29cutting off further vacuum communication to chamber 44.

Initially, the vacuum in chambers 4| vand 44 power diaphragm 48peripherallysecuredwithinr power chamber i0, assumes the position shown,with return spring 49 being interposed between valve body 24 anddiaphragm supporting plate 59. Itjwill be noted that a suitable seal48""i-s provided'around rod 4S and within .the end of valve housing 24.v

{Said 'diaphragm and plate are Vcentrally secured to hollowpower rod 46by the opposing nuts'ljand 52 threaded on said rod.

Referring to thecontrol valve body 24 it is seen that air is at alltimesmaintained in chamberh53 through A*air `passage 54 formed within Valvebodyfmembers 24and25.- y

As ,valve control pistonv moves to the left relatively torrod 46, in themanner hereafter described, it rst engages diaphragm disc 2S for cuttingoff furthervacuum `communication between chamber 42 and chamber 44.Further movement to the left of piston 31 causes transla- 'tionV to theleftf'of diaphragm y28 as well as the anged member 32 for unseatingspring retained disc k33 from its'seat 36.

" lAir in charr'iber`53` isnow Aable 'to flow through 56, the outer endof which is adju-stably secured by removable cover 51 threadedin the endof hydraulic cylinder 4.

It will be noted that rod 46 is secured at all times to piston 2| bymeans of the spring retained ball lock 58 so that piston V2| alwaysmoves with rod 4B. Ball lock 58 maybe manually disengaged from piston 2|lin the event it is desired to separate said piston and rodffor discassembly and, or cleaning. Aplug 59 is tightly secured within hollow rod46 withv a suitable passage formed therein to coincide with and form apart of passage 20am piston 2|. Ball chain 55 interconnects valve 55 andcover 51 to limit movement of said valve.

Y Referring to Figs. `1 and 2,`for conventional operation of the brakesthe master cylinder 1 is operated by foot pedal 5 for directing iiuidunder pressure through conduit 8,'into port I9 of the hydrauliccylinder, chamber 39 and through passage20 into chamber i'whence it isdirected out cylinders l for the vehicle wheels respectively. Thisinitial actuation of break pedal 5 will result in a partial brakingeifect upon wheel drums 2 and will also take up the slack of fluid inchamber 55 as well as in conduits 3.

As soon, however, as saidsslack is taken up itwill vbe seenlthatcontinued and further actuation of brake 4pedal 5 will tend to compressuid within chamber 39 as well as in chamber 6I] with the result thatvalve control piston 31 is projected to the left. In the i'lrstinstance, it will be seen that disc 41 carried by said piston will,engage the diaphragm disc 29 for closing olf further vacuumcommunication to chamber 44 forming a part of power chamber I0. It willbef noted that momentarily there is a balanced condition betweenchambers 4| and 44 with vacuum in both chambers. However, furthermovement of control piston 31 to the left eiTects movement to the leftof diaphragm 28 as Well asthe flanged member y32 which unseats springretained air'control disc'33.

Air which is'const'antly in chamber 53 is thus free to rushthrofughopening 35 in'valve seat 36 for"y communication' with chamber 44'through hollow.v rod 46.' yInstantly there is a forceful translation'ofdiaphragm 48 to the right due to the pressure differential betweenchambers4 4| and 44 which is proportionate to the surface area of saiddiaphragm, andwhich results in forceful translation of power rodf4'6.Hydraulic piston 2| eingl joined thereto istalsotranslated forcefully tothe'right, first sealing off passagev 2|] by engagement of valve member55.v Further movement vof hydraulicl pistonv2| tends to exert acompressive force jupon( the iiuid in chamber 60 with the result that asecondary power braking is effected'through lines 3. It will beunderstood that as'soon as passage 2|) is closed off by engagement withv'alve'member 55 uid under pressure, which is maintained by continuedapplication of brake pedal 5, is directed to chamber 39; and inasmuch as'passage 20 is now closed, it will be seen that a degree ofk iiuidpressure eiis'tsfor cooperative action upon the end of piston *2|working in unison with the power rod 46 which is also effecting lapressure movement on piston 2|' relativef'to the hydraulic fluid inchamber 6D. f Y.

For example, while 'the' initial braking pressure may be approximately200 pounds per square inch, power chamber I9 effects a substantialmultiplication of braking Vpressure so that approximately`800to'1000`pounds per square inch pressure is available. VIt is understoodthat the above pressure-sI are givenlrnerely by way of illustration andnaturally would be varied depending upon 'the size of power chamber llland the surface area of diaphragm 48. Movement 0I piston 2| to therightwincreasing the volume of chamber 39 partially r'educesthe existingiiuid pressure therein permitting a partial return of valve 'pisto`n31'to"1 `the right under action of coiled spring 38.v `Diaphragm 28 andplate 29, under action offspring 34, follow Valve piston 31l to theright,'permitting air disc 33 to seat itself under" action of `saidspring, 34, cutting off further air communication between chambers 53and 44. Said, valve assembly is now in a balanced condition with furtherair and Vacuum communication tochamber 44 effectively cut off.

Brake release is effected by relieving the manual pressure from mastercylinder Y' l with the result-thatthe pressure in chamber 39' is reducedpermitting control -pisto'n 31"..tdreturntlto. itsinitialnositiom'under'action of coil sprng.- As'son as control? piston31-reaches the position shown' in Eig?. 2,lvacuux'n. communicationis'again established 'between chamber 4 If and" chamber 44,vwitn'the'result tnotan equilibrium in pressure is! 'established onopposite sides of diaphragm 48. Said 'diaphragm :then returns to .itsinitial position shown in Fig. 2lunder action ofcoil spring-` 491-11?will-be noted! that V,return ot diaphragm` 48`-toitsf initial' position`also eiects a correspondingdetirn of- Apower rod 46 a`s Wellas hydraulicmotionl 2li' secured thereto.

stop` member 61 is threaded through hydraulic cylinder 4 Aand.projectsinto* clfiamber` 39 "to provide alimitat'ion nfth'eieturnmovement of piston 2li aswell ast'n'ower-'rod '46 and diaphragm48:'sered thereto'f." The Areturn of pist0n`2l to its"ir 1itial positionreslts in lthe unseating ofv valve'member SS-again'opening passage 20Vin saiiif` piston 'which communicates' with chamber 39-""Thus alloperatingu members are again'in their initial positionsr'eady forthenegt actuation ofthe brakes. y' i y that in the event of a faili'ire'of'the 'power brakinhgsystem or the air and;v vacuum control valve thereinmantieni/'ap p li tio'nf'ofthebrakes'is'still available as aboveAfterfaplication ofV the'power brakes, and on release of foot'edall;thfe'ffi'idp'ressurein contr'ol'chamber 39" is reduced approachingzero', permit'tirigl ,vali/e" control piston'4 3l' to 'gradually returnA"tofvthelrigljiterelativelyfto power rod 46, until: it 'reaches 4"itsinitial position shown in Fig. 2, At the same time, withv pressure .inonainbjet et- `anzi'opnirig 'zonstiufoiosed piston zl's'taltsmovingebaol' 'to the 'initial position shown Iin,lig`. `2 dueto thediiferentialof pressure QIQOPP'OS?? S'devthreQL l Y The. etiiinmovementof piston .2| isilimited' by. this feiern'. movement 0f.. power,rod. 46, and 'tagnilfio which .itis joined.' AS@ Short pe`...d of ,timeelansesbiorfe full vacuum is again.' sieblslldin chamber. 44.,.dephrag1iigtdsito lag" irl/returning tolits inoperative position,k as' shown inFigl`V 2,eve'n 'though under vexpansive actioni'fof spi1g 4$, This. lag`.would be transmittedtoV pis/t'onl'ldelayin'g its return,v forefiectingwcompletebrake release.

However, .it is seen'b'y the initialupressure differential betweenchambers '39 andv 60 that, on releasleofh pedal 5,',niston 21 isassisted in its return; nugtnerniofef bau chaines-5y;- iimits thereturnY movement of valve 55j guaranteeing the quifclopenin'g oflpassagelliof piston 2l.'L 4Opening` of nassag'e `2'() immediatelyeffects 'complete brake releaser` with yiiuid under pressure inchamberGil f 'rec to escape. Thus brakev release isf'loroiiiit whichi'sa'great advantage'over power brakes of differenty construction whereinmeans are not provided 'for assistingvin thebrake release function.

lower chamber I U isy advantageously secured o r formed integrally withcontrolvalve 24-25 to providea compact unit which maybepositionedanywhere on Athevehicle orr truck frame. VI-low-4 ever,` itiscontemplatedthat said powerV chamber c'old be remote'from the control*Valve with a suitablelinkage' interconnecting the diaphragm aridtlievalve actuating power rod 46;

O'i` the other handit is also'contemplated that valve actuating" rod4lioouldbe forcefullyactuat'ed' by any' other suitable power.. sourceacting yit 'will' be understood Itlwillbe understoodithat while thepowert chamber lo is shown in Fig. 2y with: a' flexible movable.diaphragm therein, it Ais contemilat'ed that saiddpower 'chamber couldbe constructed withta recip'rocable piston-therein. Creationlotpressure. differentials von opposite sides of the *pis*-l ton would, .beeffectecliexactly as `described with respect ytolFigy2.-

Fig...3 shows the combination .of hydraulic brake c5i1inderi4;withtiuid.deliveryl outlet I8,and air and .vacuumvcontrol valve Z4- 25oHowever, the

y assliownfin Fig.i2, with power 'shaft 46".;recipro-r cably...positioned therethrough effecting move-v ments ot the: fluid controlpiston 2li. Connecting .roddirprojects from piston Mire- ;ciprocably.provisioned within powerchambercylinder 63.. Rod 64isfpivotallyvjo'inedatits end 65 ,toleverSS centrallypivotedat 61 tobracket 68-l or. other. member` forming` a 'part of the vehicle frame...t 'I' Power. shaft 45 is pivotally and looselyv joined at ill to theother end` of lever 'B6-.fA YVacuum.chamber .ll iin" cylinder 63 4isjoined. tothevehicle enginefintak'e manifold lby com duit l2 :as .showndiagrammatic'ally in 4yrlig. `'3.- Conduit i2:Yalsoprovides vacuumcommunication' to the valve vacuum passage 43 shown in detail in. Fig.z.4 operating chamber is on the oth'r'ide of piston E2 is supplied airorvacuum fron'i'the control valve outlet 'M- through conduitly v It isunderstood that Valve outlet i4 communi-` cates with the interiorcharn'ber'lii-of` valve ill-viii",-v and is provided air kfrom intakeportV i4 and, orn vacuurnvfrom port 43jdependinng'unon the'r'osi.itioning of the inoi'ablevaivein'enibei withnsid valvehousingnvheretoiore describedin ,detavi'lQ l Having described myr'inventions,reference shouldbe had towtheclaims which follows forAdeterminingnthe scopefthereof, Y

I dan..

1 Tine combination, a; hydraulic. cyiinden. a valve control. pistonand'a fl'uidx'lcontrol piston with an lopening therethrough onpositelydisposed; therein providingA a, uidchamber. therebetween, there .being al.secondary fluid chamber in said cylinder .i or .delivering. fluidunder pressure.A manually `vcontrolled pressure fluid sourcejoining Saidrstuchamben adapted, forcom 1nunication with saidsecondary chamber.throgh said huid; control piston, .and forop'erative pressure engage-A IIlllii..A With .said valve control piston for eiectingx movementthereof, a power chamber, a rriovabley member. therein, means on, saidmen'iberA slidably extending through said valve` control piston andAjoined at itsl outer endv to said fluid control pistonanud-adaptedtoforcefulftranslation upon a pressure differential onopposite sides of said movable chamber,.anair and vacuum controlvalveintermediatesaid. cylinder and power chamber engageableloy saidvalve control piston for effectingsaid diierential, valve means in saidsecondary-fluidJ chamber-forclosin'g the opening in said iiuidcontrolpiston on movement thereof in one direction, and` flexiblelimiting means joined to said valve means'for'unseatinghthe isamerelative to said` member therein, means on said member slidablyextending through said valvefcontrol piston and joined atits outer endto said fluid control piston and adapted to foreceful translation upon apressure differential on opposite sides of said movable member, an airand vacuum control valve intermediate `said cylinder and power chamberengageable by said valve control piston for effecting said differential,resiliently urged valvemeans in said cylinder for closing communicationthrough said fluid control piston on movement thereof in vone direction,and flexible limiting means joined to said valve means for unseating thesame relative to said piston opening on movement thereof in the oppositedirection.

`3. The combination, a hydraulic cylinder, air and vacuum valve controland brake fluid control pistons therein, the latter pistonhaving anopening therethrough, a manually controlled pressure fluid sourceintermediate said pistons adapted for'communication through said fluidcontrol piston and for operativel pressure engagement withsaid valvecontrol piston, a power chamber, a movable member therein, means on saidmember slidably extending through said valve control piston and joinedat its outer end to said fluid control piston and adapted for forcefultranslation upon a pressure differential on opposite sides of saidmovable member, a vacuum source joining said power chamber on one sideof said movable member, an air and vacuum control valve intermediatesaid cylinder and power chamber for normally maintaining vacuumcommunication to the other side of said movable member, but adapted onmovement of said valve control piston engaging said valve for cuttingoff said latter vacuum communication and establishing air communicationthereto, resiliently urged valve means in said cylinder for closingcommunication through said fluid control piston on movement thereof inone direction, and flexible limiting means joined to said valve meansfor unseating the same relative to said piston opening on movementthereof inthe opposite direction.

4. The combination, a hydraulic cylinder, a power chamber containing amovable diaphragm centrally therein defining a vacuum chamber and anoperating chamber on opposite sides thereof, an air and vacuum controlvalve intermediate and co-axial with said cylinder and power chamberadapted to alternately provide air and Vacuum communication to saidoperating chamber, a vacuum source communicating with said vacuumchamber, a valve operating piston adapted to engage said valve and abrake uid control piston having'an opening therethrough in saidcylinder, a manuallycontrolled pressure fluid source intermediate saidpistons adapted for communication through said fluid control piston andfor pressure engagement with said valve operating piston, a plunger rodslidably provisioned through said valve and valve operating pistonjoined at one end to said diaphragm and at its other end to said fluidcontrol piston, resiliently'urged means in vsaid cylinder for closingcommunication through said fluid `control piston Aon movement thereof inone direction, and ,flexible limiting means joined to saidI valve meansfor unseating the samerelative to said piston opening on movementthereof in the opposite direction.

5. The combination,` a hydraulic cylinder, a power chamber ,containing amovable diaphragm therein vdefining a vacuum chamber and an operatingAchamber on lopposite sides thereof, an air and vacuum control valvehousing intermediate and CD-axialwith said cylinder and power chamber,`amovablevalve member therein, a diaphragm peripherallysecured withinsaid housingand,supportinggsaid valve member, the latter being adaptedtoalternately provide vacuum and air kcc,mmunication through said` housingto said operating chamber, avacuum source communieating with said vacuumchamber, a valve operating .piston adapted to operatively engage saidmovable valve member, and a iluid control piston having Lan opening.,therethrough both in said cylinder, a manually controlled pressuregfluidsource intermediatesaid pistons adapted for com-- munication throughsaid fluid control piston and for pressure engagement with said valveoperating piston, a plunger rod slidably provisioned through saidhousing, movable valve member, and valve piston,joined` atione Aend tosaid rst diaphragm and at its other end to said fluid control piston,resiliently urged valve means in said cylinder for closing communicationthrough said fluid control piston on movement thereof in one direction,..and flexible limiting means joined to said valve means for unseatingthe same relative to said .piston opening on movement thereof in theopposite'direction. Y

. 6. The combination, a hydraulic cylinder, a fluid control pistonhaving an opening therethrough and a valve control piston in saidcylinder in spaced relation, a manually operable pressure uid source4.between said pistons adapted for communication through said pistonopening, and for operative pressure engagement with the other piston, apower chamber, a movable memr. ber therein adapted to translation upon apressure differential on opposite sides thereof, a longitudinallymovable plunger rod joining said movable member'at one end and saidfluid control piston at its other end and slidably disposed through saidvalve control piston, an air and vacuum control valve co-axial with and`intermediate said cylinder and'power chamber engageable by said valvecontrol piston for effecting said differential, resilient valve means insaid cylinder for closing the opening in said fluid control piston onmovement thereof in'one direction, flexible limitingmeans joined to saidvalve means for unseat-ing the same .relative to said piston opening onmovement thereof in theopposite direction, resiliently .urged meansintermediate said valve control pistonA and the end of said cylinder.

7. The combination, a hydraulic cylinder, valve control and uid controlpistons oppositely disposed therein providing a fluid chambertherebetween, said fluid control piston having an opening therethroughthere being a secondary fluid chamber in said cylinder for deliveringfluid under pressure, a manually `controlled pressure uid source joiningsaid rst chamber, adapted for communication with said secondary chamberthrough said fluid control piston, and for operative pressure engagementwith said valve control piston for effecting movement thereof, a powerchamber, a movable membertherein, means on said member slidablyextending through said e valve control piston and joined to said fluidcontrol piston and adapted to forceful translation upon a pressuredifferential on opposite sides of said movable member, an air and vacuumcontrol valve intermediate said cylinder and movable member engageableby said valve control piston for effecting said differential,resiliently urged valve means in said secondary chamber for closingcommunication through said iiuid control piston on movement thereof inone direction, and exible limiting means joined to said valve means forunseating the same relative to said piston opening on movement thereofin the opposite direction.

8. The combination, a power chamber having a movable member thereindefining an operating chamber and a vacuum chamber, a hydraulic cylindercontaining a brake fluid control piston and a valve control piston, amanually controlled pressure fluid source intermediate said pistons.

adapted for communication through said uid control piston and foroperative pressure engagement with said valve control piston, a valvehous- ,ing having air and vacuum inlet ports, and an outlet portcommunicating with said operating chamber, a hollow reciprocable valvemember in said housing, a diaphragm peripherally secured to said housingand centrally carrying said movable member providing a normally closedair passage between said air port and said outlet port, said valvecontrol piston being normally spaced from said valve member providing anormally open vacuum passage between said vacuum port and said outletport, whereby progressive movement in one direction of said valvecontrol piston closes said vacuum passage and further movement thereofcauses movement of said valve member to open said air passage, andpartial movement of said valve piston inthe opposite direction permitsclosing of said air passage, and further movement in the same directionopens said vacuum passage, whereby vacuum and air communication are-alternately provided to said operating chamber, a Vacuum sourcecommunieating with said vacuum chamber and with said vacuum inlet port,and a plunger rod secured to said power chamber movable member at oneend 10 and to said fluid control piston at its other end, slidablyprovisioned through said valve housing, said valve member, and throughsaid valve operating piston.

9. A power unit comprising a hydraulic cylinder containing uid to besupplied under varying pressures to the wheel brake cylinders of avehicle, a piston with an opening therewith in said cylinder operable onsaid fluid, a manually operable pressure fluid source communicating withsaid ud, a power chamber, a movable member therein, means on saidmovable member joined at its outer end to said piston and adapted totranslation upon a pressure dierential on opposite sides of said movablemember, an air and vacuum control valve co-axial with and intermediatesaid cylinder' and power chamber for effecting said diierential, asecondary piston in said hydraulic cylinder in spaced relation to saidrst piston y engageable with said valve and operable upon by saidmanually operable pressure fluid source for controlling said valve,valve means in said cylinder for closing the opening in said rst pistonon movement thereof in one direction, and flexible limiting means joinedto Said valve means for unseating the same relative to said pistonopening on movement thereof in the opposite direction.

GORDON R. ELLIOTT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Rockwell Mar. 20,

